This invention relates to a lubricant composite for powder metallurgy and to the manufacture and use of this lubricant composite. More particularly the invention concerns a lubricant composite including a combination of at least two lubricants.
Powdered metals, for example, powdered iron, are used to make small, fairly intricate parts, for example, gears. The fabrication of such metallic parts by powdered metal technology involves the following steps:
the powdered metal is blended with a lubricant and other additives to form a mixture,
the obtained mixture is poured into a mould and compacted to form a part using a high pressure, usually of the order of 200 to 1000 MPa,
the part is ejected from the mould and subjected to a high temperature to decompose and remove the lubricant,
the part is heated to a higher temperature to cause all the particles of metal in the part to sinter together and,
the part is cooled, after which it is ready for use.
Lubricants are added to metal powders for several reasons. One reason is that they facilitate the production of compacts for sintering by lubricating the interior of the powder during the compaction process. Through selection of proper lubricants higher densities, which is often required, can be obtained. Furthermore, the lubricants provide the necessary lubricating action that is needed to eject the compacted part out of the die. Insufficient lubrication will result in wear and scuffing at the die surface through the excessive friction during the ejection, resulting in premature die failure. The problems with insufficient lubrication can be solved in two ways; either by increasing the amount of the lubricant or by selecting more efficient lubricants. By increasing the amount of lubricant, an undesired side effect is however encountered in that the gain in density through better xe2x80x9cinternal lubricationxe2x80x9d is reversed by the increasing volume of the lubricant. The better choice would then be to select more efficient lubricants.
The known high effective lubricating agents however have low melting points. This distinguishing feature results in problems already before the compaction process as regards the powder flow and the apparent density. A relatively free powder flow is essential for smooth operation in a production press, while a stable apparent density facilitates a high quality during the production. The parts are thus of equal weight and exhibit tight dimensional tolerances, reducing the need for post operations such as calibrations.
The use of very efficient lubricating agents have thus until now been limited due to their negative impact on powder properties. An object of the present invention is to provide a process for making these lubricants industrially useful.
In brief the process for making the new lubricant composites according to the invention includes the steps of
selecting a first lubricant having a melting point or a substantial part of its melting below 110xc2x0 C. and a second lubricant having a melting point or a substantial part of its melting above 120xc2x0 C. ;
mixing the lubricants at an elevated temperature in order to melt the lubricants and
subjecting the mixture to rapid cooling for providing a metastable lubricant composite.
Examples of lubricants within the first group are saturated and unsaturated fatty acid amides and bis-amides, such as stearamide, oleamide and ethylene-bis-oleamide. The amount of this first lubricant depends on the specific lubricant and may vary between 5 and 75% by weight.
The second lubricant may be selected from lubricants presently used in powder metallurgy and preferably this lubricant is selected from the group consisting of fatty acid bis-amides, such as ethylene-bis-stearamide (EBS).
The mixture of the two types of lubricants are heated during mixing at a temperature above the melting point of the second lubricant for a time period sufficient to provide a homogenous mixture, which is then subjected to a rapid cooling which is a critical feature of the process according to the present invention.
The rapid cooling rate, can be achieved by several well-known methods, such as through pouring of the melt into liquid nitrogen or water, by atomisation of the material from the melt or by pouring the melt onto a cooled metal surface. The cooling rate necessary is dependent on the composition and may also vary with the relative amounts of the first and the second lubricant. For example, cooling rates above 100xc2x0 C./s may be necessary for some compositions and amounts, whereas cooling rates about 1xc2x0 C./s may be sufficient in other circumstances. In any case accelerated or forced cooling is necessary in order to achieve the metastable phase which is a distinguishing feature of the new lubricant composite according to the present invention and which makes it possible to take advantage of the valuable lubricating properties of relatively low melting lubricants, which in the form of the metastable phase retains the high lubricating effect but looses the negative influence on the flow.
Depending to the mode of preparation the solidified lubricant composite may then be disintegrated to a suitable particle size by e.g. milling. Preferred average particle sizes are between 3 and 150 xcexcm.
A spherical shape is the most desirable, because this leads to the highest flow rates and apparent density When mixed with metal powders, the concentration of the lubricant composite plus optional conventional solid lubricants, is suitably in the range of 0.1 to 5% by weight, preferably from 0.3 to 1% by weight.